Semiconductor laser devices have been widely used in optical disc systems and optical communications (see, e.g., Japanese patent publication No. JP-3080312, JP-A-2002-100830 and JP-A-2004-296903). These semiconductor laser devices include a cavity to generate a laser beam. The cavity has a front facet formed on one end thereof to emit the laser beam and has a rear facet formed on the other end. An insulating film, or coating film, is formed on each facet to reduce the operating current of the semiconductor laser, to prevent return of light, and to increase the output power.
Generally, in high power semiconductor lasers, a low reflectance coating film is formed on the front facet, and a high reflectance coating film is formed on the rear facet. Specifically, the reflectance of the coating film on the rear facet is typically 60% or higher, preferably 80% or higher. The reflectance of the coating film on the front facet, on the other hand, is determined based on the required characteristics of the semiconductor laser. (Lower reflectance does not necessarily guarantee higher laser performance.) For example, semiconductor lasers for exciting a fiber amplifier used in combination with a fiber grating employ a coating film having a reflectance of approximately 0.01-3%. General high power semiconductor lasers, on the other hand, employ a coating film having a reflectance of approximately 3-7%, or a coating film having a reflectance of approximately 7-20% when it is necessary to prevent return of light.
The coating films protect the facets, that is, they function as passivation films for the semiconductor interfaces (see, e.g., Japanese patent publication No. JP-A-2004-296903 noted above). However, coating films of conventional material have a problem in that an interface state occurs at their interface with the underlying semiconductor, resulting in a reduced COD (Catastrophic Optical Damage) threshold.
To prevent such COD threshold reduction, a GaAs semiconductor laser may have a window structure formed by disordering the active layer, which has been proven to be effective and has been practically implemented (see, e.g., Japanese patent publication No. JP-A-2006-147814). Further, efforts are being made to form such a window structure in a nitride semiconductor laser.
For reference, the followings are prior art Japanese patent publications.
JP-3080312,
JP-A-2002-100830,
JP-A-2004-296903,
JP-A-2004-296903,
JP-A-2006-147814
However, blue semiconductor lasers also have another problem with regard to the coating films. Since the wavelength of the laser beam of these semiconductor lasers is short, the coating films have a high optical absorption coefficient and hence tend to degrade, as compared to conventional semiconductor lasers. Further, since conventional coating films do not adequately function as passivation films, the crystalline structure of the facet portions of the laser may be degraded, resulting in a reduced COD threshold.
The present invention has been devised to solve the above problems. It is, therefore, an object of the present invention to provide an extended life semiconductor laser device in which a protective film is formed on one or both of the facets of the cavity.
Other objects and advantages of the present invention will become apparent from the following description.